1. Field of the Invention
The invention is directed to a cylinder piston device and more particularly to a mono-tube vibration damper, including a cylinder having an inner wall bounding a work space filled with a working medium, such as hydraulic fluid. A closure is fixed axially with respect to the cylinder by position engagement with the cylinder. The closure bounds the work space and is sealed with respect to the inner wall by a circumferential seal, but is axially movable with respect to the cylinder when a temperature threshold is exceeded.
2. Description of the Related Art
It is a characteristic of a mono-tube vibration damper that it may be subject to extreme temperature loading, for example, in case of an automobile accident in which the mono-tube vibration damper of the motor vehicle is exposed to fire. In such a situation, the work medium found in the piston device, typically a damping fluid, undergoes a very great expansion, so that a pushing-out force acting on the piston rod and a closure at an axial end of the piston rod overloads the fastening of the closure within the cylinder, the fastening being designed for normal operation. In a worst case scenario, the piston, together with the piston, may be explosively pushed out of the cylinder, due to the pressure level within the mono-tube vibration damper. This can lead to devastating consequences, including injuries to passengers, for example, when the mono-tube damping device is used as a shock absorber in a passenger automobile.
Solutions have been suggested to ameliorate this scenario. These solutions have in common that they attempt to ensure a controlled drop in pressure, rather than an explosion. Some known solutions provide for a deliberately weakened area in the seal to allow for escape of pressurized fluid under extreme pressure situations.
In DE3901449, a solution is proposed in which the seal of the tube is formed so as to have an element that melts upon occurrence of high temperatures, to relieve pressure in the interior. This solution works by providing a deliberately meltable portion of the seal.
U.S. Pat. No. 5,454,455 discloses that at least one bead provides a stop surface for the piston rod-piston unit to prevent its expulsion. However, this solution requires a specially adapted piston and, in some cases, may lead to a loss of useful stroke because the bead is formed within the travel path of the piston.
U.S. Pat. No. 6,840,153, commonly assigned with the present application, discloses a piston-cylinder unit in which a sealing closure is fixed axially by a positive engagement with the cylinder up to a temperature threshold. The closure can carry out an axial movement for relieving pressure above the temperature threshold, in which the relief of the pressure is achieved by providing a cutting device as a tongue-like projection from the wall of the cylinder. Upon axial movement, the cutting device performs a cutting action that cancels the sealing function of the seal. However, this solution requires providing additional structure, i.e., the cutting device, whose only purpose is providing the channel in the high temperature situation.
The danger of uncontrolled explosive axial movement of the piston seal has also complicated and increased the costs of transporting mono-tube vibration dampers. In particular, in some jurisdictions, particularly in the United States, mono-tube vibration dampers are treated, for purposes of transportation, in the same manner as explosives, limiting access to certain roads, bridges and tunnels, increasing the cost of transporting these products. For example, the characteristics discussed above have meant that only certified transportation companies are permitted to transport mono-tube dampers, and only designated roads and border crossings must be used.